Feb., 1952
RACEMIZATION STUDIES ON TRIS-(2,2’-DIPYRIDYL)-NICKEL(II)
CHLORIDb
195
RACEMIZATION KATE STUDIES OX TXIS-(~,~’-DIPYRIDYL)-NICli;EL(II) CHLORIDE BY GEO.K. SCHWEITZER AND JOHN M. LEE Department of Chemistry, University of Tennessee, Knoxville, Tenn. Receired December 6 , 1860
The complex compound tris-(2,2’-dipyridyl)-nickel(II)chloride was prepared. Resolution of this compound ?as then 01)tained by fractional crystallization with ammonium d-tartrate. The molecular rotation of 4294 degrees at 5893 A. and 23.5’ compares favorably with values reported previously for the hexahydrate. The rates of racemization iii water were determined in the temperature range 16.5’ to 40.0’. The energy of activation for this racemization was calculated t o be 21,900 cal. per mole. Excess dipyridyl has no effect upon the rate of racemization, but excess nickel (11)ions had a slight accelerating influence. These results tend to discount the ionization mechanism theory of racemization. Acid and base produced nlnioRt identical accelerating effects. It is probable that this merely means that the active complex is most. stable in neutral or very slightly acid solutions. The effects upon the racemization rates by mixed solvents were noted. In the acetone-water, dioxmewater, methanol-water and ethanol-water systems, a non-linear retardation of the rate with varying concentration o f the non-aqueous solvent was obtained. Activation energy calculations gave no indication of a change in mechanism.
Relatively few investigators have undertaken racemization rate studies on optically active inorganic coordination compounds in solution. The effect of temperature on a number of racemization rates has been reported. Several studies of the effects of added ions on the rates appear in the literature. Only two authors report any observations on racemizations in mixed solvents. Werner’ noted that a solution of 7 parts acetone to three parts water had a retarding effect upon the rate of racemization of potassium tris-(omlato)-chromate(111). Bushra and Johnson2 also found that acetone in aqueous solution had a retarding effect upon the racemization of the potassium salts of tris- (oxalato)-chromate (111) , tris-( oxalate)-cobaltate(II1) and bis-(oxa1ato)-ethylenediaminechromate(II1). They made comparisons of rates only at 0.000, 0.059, 0.098 and 0.144 mole fractions of acetone. In view of the small amount of work that has been reported, it was proposed to explore some of the factors influencing the rate of racemization of another optically active coordination compound. The complex that was selected was tris-(2,2’dipyridy1)-nickel(I1) chloride because of its ease of resolution, low optical density, high initial rotations in dilute solution, and favorable racemization rate. It was proposed to observe the effects of the following variables upon the racemization rate: excew of common constituents, temperature, hydrogen and hydroxide ions, and mixed solvents. Preparation and Resolution.-Blaua first reported the preparation of tris-(2,2dipyridyl)-nickel(11)chloride. With some modifications, his method was used by Morgan and Burstall‘ and Jaeger and van Dijk.6 The rocedure adopted in this research waa slightly different t i a n any of these methods. The difficultly soluble 2,2‘-dipyridyl reacts readily with nickel(I1) chloride hexahydrate in the presence of a slight amount of water. To 4.46 g. of 2,2’dipyridyl and 2.27 g. of nickel(I1) hexahydrate, 8 ml. of water was added. A clear, deep red solution resulted when the mixture was warmed. Upon cooling, pink-violet plates crystailized in good yjeId. The product was atered with suction and then dried in a vacuum desiccator over concentrated sulfuric acid for 24 hours. The yield of tris-(2,2’dipyridy1)-nickel(I1) chloride waa 5.11 g. As previously reported by Morgan and Burstall4 and Jaeger (1) Werner, Ber., 45, 3081 (1912). (2) Bushra and Johnson, J . Chem. Soc., 1937 (1939). (3) Bhu, Monatsh.. 19,668 (1898). (4) Morgan and Burstall, J . Chem. SOC.,2213 (1931). (5) Jaeger and van Dijk. 2. anorp. allpem. Chem., *ST, 304 (1936).
and vanDijk6ammoniumd-tartratc was employed as the resolving agent. It was found, however, that more consistent resolutions were obtained by starting with d-tartaric acid. Neutralization of 98 g. of d-tartaric acid by adding 140 ml. of concentrated ammonium hydroxide gave the proper concentration of ammonium d-tartrate when the resulting ~ o l u tion solution was diluted with 80 ml. of water. This solution was warmed to expel excess ammonia until it was just slightly basic as tested with alkacid paper. To the warm solution, 5.1 g. of tris-(2,2‘-dipyridyl)-nickel(II) chloride was added. The solution was then nicchanically stirred for one hour. After checking the activities of fractions which were removed at various temperatures as the solution was allowed to cool, it was found that the more active fractions had crystallized upon reaching 20”. The escess animonium d-tartrate collected in a separate layer in the bottom of the beaker. With slight agitation, the active tris(2,2’-dipyridyl)nickel(II) d-tartrate could be suspended in the solution, decanted from the ammonium d-tartrate, and filtered with suction. The precipitate was taken up in as little cold water as possible and the tris-(2,2’-dipyridyl)nickel(I1) chloride was precipitated inimediatcly with concentrated ammonium chloride solution. Solution and reprecipitation of the salt was carried out again. Very little. racemization takes placc if these steps arc carried out at a temperature near the ice point. The light pink active form was then dried over Concentrated sulfuric acid in a vtlcuuin desiccator with constant pumping. After oiic hour, the very dry product was removed and placed in a tightly sealed screw-lid bottle. Under such conditions, the active complex may be kept for an almost indefinite pcriod. On(, sample so prepared showed no appreciable loss in rotary power after six weeks. Apparatus.-The polarimeter used in this investigation was a Joseph and Jan Fric model yhich is equipped with a vernier to read directly to 0.01 A circulating pump made by the American Instrument Company was employed to force water from a constant temperature bath through a jacketed 2-dm. polarimeter tube of 11-ml. capacity. Thc required low temperatures were obtained through the use of cooling coils from a standard refrigcrating unit, while a Sargent heating element, with a mercyry thermostat provided temperature control within 0.1 . A sodium vapor lamp supplied by the G. W . Gates Company providcd thv polarimeter with a monochromatic light source. Racemizations in Water .-Solutions of the complex werc made with sample weights held close to 50 mg. in 15 nil. This volume gave a slight excess over the amount required for the polarimeter tube, while the weight of complex gavv initial rotations of from 3 to 5 ’ , depending upon .thv temperature and the time elapsed before making the initial reading. In aqueous solution, the complex was found to undergo racemization at an over-all rate indicative of a first order reaction. Sample curves are shown in Fig. 1 to indicate the general quality of the data. All rate calculationn were made by the method of least squares. By using samplc weights of from 35 to 110 mg. in 15 ml., it was shown that the racemization rate in this concentration range is independent of the concentration of the complex. With the concentrations indicated, checks in rates werc obtained to within 0.1 minute.
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GEORGE I
